1. Field
One or more example embodiments relate to a method and apparatus for monitoring a state of consciousness, and more particularly, to a method and apparatus for monitoring a state of consciousness of a human being or a patient in response to administration of a drug.
2. Description of Related Art
A surgery patient or a critically ill patient may need an appropriate level of hypnosis to prevent stress from a surgical operation, or awareness or memory of the surgical operation, and thus rapid and accurate determination of a depth of anesthesia may be important and essential in managing or controlling anesthesia during and after the surgical operation.
In a process of conducting a medical treatment for a patient, a method of anesthetizing the patient may be classified into inhalation anesthesia and intravenous anesthesia. For the intravenous anesthesia, a sedative and an analgesic may be used simultaneously. However, administering an excessive dose of such anesthetic drugs to a patient may leave the patient being in a serious condition, for example, a respiratory failure and a drop in heart rate and blood pressure, and thus monitoring a state or a condition of the patient and administering an appropriate dose of an anesthetic drug may be needed.
To determine a minimum dose of an anesthetic drug having a sufficient anesthetic effect on a patient, quantitative measurement of an anesthetic effect may be needed. For the inhalation anesthesia, a single drug having both a sedative effect and an analgesic effect may be used to anesthetize a patient. However, the effects may not be individually adjustable. In contrast, in a case of the intravenous anesthesia, a sedative and an analgesic may be individually administered, and thus adjusting a concentration of each of the sedative and the analgesic may be possible as necessary. The two anesthesia methods described in the foregoing may be used selectively or by mixture depending on a state of a patient. Thus, for effective anesthesia, how each of a sedative and an analgesic affects a patient may need to be measured quantitatively.
A bispectral index (BIS) is generally used as an index indicating a sedative effect of an anesthetic drug that is currently used during a surgery to measure a state of consciousness of a patient using a frequency and a phase of a brainwave.
The BIS may be a simplified numerical value indicating a level of sedation and hypnosis induced by an anesthetic or a sedative, and determined by measuring a frequency, amplitude, and coherence in an electroencephalogram (EEG). The BIS may be indicated as a score from 0 to 100, which indicates a state from an alert state to an unconscious state, through an algorithm-based analysis and processing performed on a degree of a change in an electrical activity of a cerebral cortex that may be induced by anesthesia. In detail, the BIS may be a method of analyzing a signal detected from an electrode and indicating a level of sedation of an anaesthetized patient as a score out of 100. Here, 100, a highest score, may indicate a fully conscious state and 0, a lowest score, may indicate an unconscious state with a flat-lined EEG indicating that a brainwave is not completely detected.
Recently, a probability of an error in a BIS monitor has continued to be reported. It is reported that approximately 14 seconds to 155 seconds are delayed for updating a BIS due to a complex anesthetic depth algorithm of the BIS monitor. Such a delay in calculating a BIS may demonstrate a limitation of the BIS monitor as a patient monitoring device to prevent the patient from being alert or awake and forming memories during a surgical operation.
An anesthetic depth monitoring device of a General Electric (GE) company may assess a depth of anesthesia by using an index, for example, a spectral entropy (GE Healthcare, Helsinki, Finland) based on a brainwave and an electromyogram (EMG), and presenting two types of values, for example, a state entropy (SE) and a response entropy (RE). However, there may be a high probability of an error in such a device due to an influence of using a muscle relaxant.
In addition, the BIS monitor and the GE device may not consider a functional connectivity between different cerebral cortices. For example, according to recent papers entitled “Cortical hypersynchrony predicts breakdown of sensory processing during loss of consciousness” published in Current Biology 21, 1988-1993 (2011), by Supp G G, Siegel, M., Hipp J. F., and Engel A. K, entitled “Tracking brain states under general anesthesia by using global coherence analysis” published in Proceedings of the National Academy of Sciences (PNAS) 108, 8832-8837 (2011), by Cimenser A. et al., and entitled “Electroencephalogram signatures of loss and recovery of consciousness from propofol” published in PNAS 110, E1142-E1151 (2013), by Purdon P. L. et al., an increase in connectivity in a prefrontal or frontal region may have a close correlation with loss of consciousness and a decrease in a level of consciousness. However, descriptions in the papers may not be applicable to an anesthetic depth monitoring device because the papers describe a need of 128 sensors with a high-resolution EEG to quantify the functional connectivity.
For another example, a related art, US 2013/0245485 A1, discloses technology for measuring a depth of anesthesia based on an intensity or strength of a connectivity between brainwave signals by determining a directional feedback connectivity and monitoring a feedback activity of a patient. However, according various research papers, for example, some papers published in Anesthesiology 119, 1347 (2013), by H. Lee, G A. Mashour, G J. Noh, S. Kim, and U. Lee, and published in Public Library of Science (PLoS) One 7, e46313 (2012), by U. Lee, H. Lee, M. Müller, G J. Noh, and G A Mashour, a strength of a connectivity between signals of cortices may increase or decrease after anesthesia. As described above, there are various research results associated with a correlation between a strength of a connectivity between brainwave signals and a depth of anesthesia. Thus, a consistent conclusion about an anesthetic state may not be drawn from a strength of a connectivity between brainwave signals, and thus there is still a limitation in using information associated with a strength of a connectivity between brainwave signals as an index of a depth of anesthesia.
In addition, the related art, US 2013/0245485 A1, discloses only a technology for assessing and determining a functional connectivity between brainwave signals using a phase lag index (PLI) and a directed phase lag index (dPLI) obtained by applying a Heaviside step function to the PLI. However, such technology may have a limitation in accurately assessing a complexity of a connectivity between brainwave signals, which will be described later in the present disclosure.
For still another example, another related art, KR 2009-0115306 A1 and U.S. Pat. No. 7,228,169 B1, discloses a method of determining a level of consciousness of a patient by comparing a measured signal and a reference signal. However, using only a single signal may not be sufficient to determine a complexity of a connectivity between brain regions.
As described above, assessing a depth of anesthesia may be highly significant for a surgery requiring general anesthesia, and thus numerous studies are being conducted in order to objectively assess a depth of anesthesia. However, a complete anesthetic depth monitoring device is yet to be developed, and a device may only monitor each level of consciousness.